It does not take cold weather to cause hypothermia. I saw someone start into hypothermia in mid-summer on the Nantahala River in North Carolina. The Nanty is a bottom of the dam release river, the water is cold, and the river is in a gorge (hence the name, Land of the Noonday Sun!) Wearing cotton, the individual was only in the water a few minutes, but was shaking and turning blue in that little time. I never wear cotton on the water. I tend to dress warmer, and in cold water, no matter the outside temperature, wear fuzzy rubber shorts or pants and often my neoprene vest over a capilene shirt. I would rather sweat than get chilled and ruin my paddling day.